1. Field of the Invention
This invention relates to a process and to apparatus for achieving frame synchronization in a pulse code modulation (PCM) receiver of a PCM time division multiplex (T.D.M.) telecommunications network with the aid of frame synchronizing signals which occur in each m-th of the pulse frames on which the telecommunications network is based, in particular in each second pulse frame, wherein a hunting process is carried out during a synchronizing time interval following the occurrence of frame synchronizing signals spaced by m pulse frames, and as a result of this event, the synchronous state of the receiving counter is established. A procedure of this type complies with the CCI recommendations inasmuch as a hunting process for genuine frame code words from which the aforementioned frame synchronizing signals are derived is considered to be terminated when, following the first reception of a frame code word at a time interval of one pulse frame, the frame code word fails to appear and a frame code word is received again at an interval of one further pulse frame.
2. Description of the Prior Art
Circuit arrangements have already been proposed which carry out a process of this type with a relatively low circuit expense. In this connection, one may refer to the German Patent Applications Nos. P 25 43 389.9 and P 25 42 868.6.
In the aforementioned circuit arrangements, the device which is to be synchronized is set the first time a pulse frame code word is established. When, one pulse frame later, a pulse frame code word again occurs or, two pulse frames later, no pulse frame code word is received, a new hunting process is immediately carried out which likewise involves a resetting of the device to be synchronized. In order to avoid that, following the conclusion of a hunting process and the failure of a frame code word to appear at the expected time, the occurrence of each further frame code word will lead to a resetting of the device to be synchronized, in the aforementioned proposed circuit arrangements, the use of time elements ensures that the setting of the device to be synchronized which is carried out following the conclusion of the hunting process is maintained at least over n and preferably 6 pulse frames which satisfies a further requirement of the aforementioned CCI recommendations.
The simulation of a frame code word by speech signals occurs relatively frequently. In most cases, the frame code word is then received over a large number of pulse frames with the result that the hunting process for a correct frame code word lasts for a plurality of pulse frames. In order to avoid this, a process has been proposed in which the contents of the time sections within which frame synchronizing signals can occur is stored for one pulse frame, and is compared with the contents of the section, corresponding time-wise, of the particular following pulse frame, and wherein the first signal to occur during the synchronizing time interval when non-identity has been established, resets the receiving counter into a determinate starting position.
This ensures that simulated frame code words of this type which appear over a period of several pulse frames in the same time slot can in no way lead to the setting of the device to be synchronized. Only when, following the reception of a frame code word in the same time slot of the following pulse frame, the frame code word fails to appear is the device to be synchronized set into a determinate starting position. Therefore, incorrect setting of the receiving counter can only be brought about by simulated frame code words which can be recognized at an interval of more than two pulse frames. Here again, however, it is necessary to ensure, by means of additional circuit measures, that, following the establishment of the synchronous state, the existing setting conditions can be changed only when the frame code word has failed to appear several times. However, this also means, of course, that a new hunting process for correct frame code words does not begin until a predetermined time interval of this type, for example 6 pulse frames, has expired.